The gate-to-source voltage for a field effect transistor affects the transistor's on-resistance. To achieve constant conductance operation, the gate-to-source voltage should be held constant while the transistor is on as practiced by a bootstrapped switch circuit having a bootstrapped switch transistor used to sample an input voltage. Prior to a sampling period, a positive terminal of a bootstrap capacitor is charged to a power supply voltage with respect to a negative terminal of the bootstrap capacitor. During the sampling period, a loop circuit is formed by the switching on of a pair of loop transistors in the bootstrapped switch circuit. One of the loop transistors allows the input voltage to flow to the negative terminal of the bootstrap capacitor. Since the bootstrap capacitor was already charged to the power supply voltage, the positive terminal voltage is boosted to the power supply voltage plus the input voltage (ignoring any losses). This elevated voltage may be denoted as the bootstrapped voltage.
The switching on of a remaining one of the loop transistors allows the bootstrapped voltage to charge the gate of the bootstrapped switch transistor. The gate-to-source voltage for the bootstrapped switch transistor thus stays approximately equal to the power supply voltage. In this fashion, the bootstrapped switch circuit can accurately sample the input voltage due to the constant conductance operation despite variations in the input voltage.
The gate of the bootstrapped switch transistor may have a relatively high amount of parasitic (loading) capacitance. This capacitance slows the switching on (the activation) of the loop circuit. When the sampling period is over, this same capacitance slows the deactivation of the loop circuit. The resulting loading slows the sampling speed of the input voltage.
Accordingly, there is a need in the art for bootstrap switch circuits with improved switching speeds.